1. Field of the Invention
The invention relates to double valve assemblies used for safety purposes in connection with industrial equipment such as presses, and more particularly, to means for indicating discrepant positions between the two valves of the double valve assembly. Double valve assemblies of this type are disclosed in Di Tirro et al U.S. Pat. No. 2,906,246. Mahorney et al U.S. Re. Pat. No. 28,520, Sweet U.S. Pat. No. 3,757,818 and Cameron U.S. Pat. No. 3,858,606. They are of two general types. In the first type, two three-way valves are so arranged that both inlet and exhaust flows to and from the working port are in parallel through the two valves. The second type is so arranged that the inlet flow is in series through the two valves but the exhaust flow is in parallel. In both cases the valves are piston operated and controlled by solenoid-operated pilot valves.
2. Description of the Prior Art
It is known to control switch means in accordance with the positions of the two valve stems, the switch means controlling an indicator or other safety or anti-repeat circuits which will de-energize the solenoid and stop the press. Such switch means is shown, for example, in Mahorney et al U.S. Re. Pat. No. 28,520. This patent shows piston-operated normally closed main control valves for a double valve assembly, controlled by normally closed solenoid-operated pilot valves. The switch means, however, is so arranged as to be operated when the three-way valves are in a partially or fully open position. Such a position is relatively unsafe since when the valves are open, they normally have actuated an air-operated clutch which drives the press through its cycle, whereas when the valves are closed (in their exhaust position), the air cylinder which operates the clutch and brake is also exhausted, thus applying the brake and holding the press at the top of its stroke.
Other somewhat less pertinent patents in this field are Ruchser U.S. Pat. No. 3,139,109 and Herion German Pat. No. 1,057,209, published May 14, 1959.
A double valve assembly manufactured by Bellows-Valvair, Akron, Ohio (Bulletin CP3082, Model Nos. 219700-01) does have switch means so located as to be actuated when the two main control valves reach their closed or safe positions, but the Valvair main control valves are constantly urged to their open positions, the solenoid-operated pilot valves being open when the solenoids are de-energized so that the main valves are held closed. The main valves however, since they are constantly urged to their open positions, are considered relatively unsafe or unstable as compared with normally closed main valves since, without piston pressure being applied to the main control valves, they will revert to an open position, making it possible to have inadvertent operation of the air cylinder or the press clutch.